1. Field of the Invention
This invention is in the field of integrated circuits and more particularly those circuits using switched capacitors for providing electrical equivalents of certain of the resistive circuit elements.
2. Description of the Prior Art
Switched capacitor circuits for filtering, herein described as "SCF circuits", that are integrated in a monolithic stratum such as silicon are known in the art and discussed, for example, in the following articles:
"MOS Switch-Capacitor Filters"; Robert W. Brodersen, Paul R. Gray, and David A. Hodges; Proceedings of the IEEE, Vol. 67, No. 1, January 1979, pp. 61-74. PA0 "First monolithic PCM filter cuts cost of telecomm systems"; Harian Ohara, Stewart Sando, Paul Gray, Dan Senderowicz; Electronic Design 8, Apr. 12, 1979, pp. 130-135.
Briefly, such switched capacitor filter circuits are active type circuits allowing input signals in certain frequency ranges to be transmitted through the filter with some predetermined gain or loss while preventing or minimizing the transmission of signals in certain other frequency ranges. In conventional, and non-switched capacitor, type active filter circuits, the inductive, capacitive, and resistive frequency selective networks which are normally used in passive type filters, are replaced with one or more active devices, such as operational amplifiers, in combination with associated capacitive and resistive networks. In switched capacitor filter circuits, such as described in the references above, certain of the resistive circuit components are replaced with respective switched capacitors and the electrical filter characteristics are a function of capacity ratios as opposed to conventional active and passive filters where the frequency characteristics are a function of the actual values of the inductance, capacity and/or resistance used in the filter networks.
Conventional active type filter circuits, by eliminating the requirement for the use of bulky inductive circuit components, are advantageously suited for implementation in integrated circuit form; however the requirement for the use of resistive circuit components is not ideal, especially where high circuit component density or tight tolerances on the resistance values are a necessity such as may for example be encountered in large scale integrated circuits and or in filters where the filter characteristics must be closely controlled. Since the requirement for resistive circuit components in the frequency controlling networks are for the most part eliminated in the prior art switched capacitor type filters, such type filters tend to overcome the problem associated with conventional active filters but only to a limited degree. Although the control of integrated circuit capacity elements are in general more easily controlled in value than are resistive type elements, the control of capacity becomes more difficult as the value of capacity decreases. Thus, there exists a minimum capacity value of an integrated circuit capacity element below which value the capacitor is not practical to make and/or the value cannot be controlled within a desired tolerance. This limitation in the prior art switched capacitor filters prohibits maximum use of the integrated circuit or monolithic stratum especially where filter circuitry is required having tightly controlled filter characteristics. This less than optimum use of the prior art switched capacitor integrated circuit "real estate" results in higher production costs and physically larger circuits.
The filter characteristics of a switched capacitor filter, as previously stated and as is well known in the art, is a function of capacity ratios. Thus, for example, the capacities in a given ratio "R" can be proportionally reduced in capacity value without either changing the ratio "R" or the particular filter characteristic which the ratio controls. The smaller of the capacities however cannot be reduced to a value lower than the smallest practical capacitance that can be fabricated or integrated on the monolith. This capacity is identified herein as a "minimum unit of capacity", or a "MUC"; it is the minimum amount of capacity which is practical to fabricate or integrate on a monolithic or film stratum and which can be controlled within desired capacity tolerances in production quantities. Obviously the actual capacitance value of a MUC will be a function of the fabrication techniques and can change for example as the monolithic state of the art changes.
"Total circuit capacitance" and "circuit capacitance" are defined herein as the actual capacitance that must be fabricated on the monolithic or film stratum to obtain the desired circuit characteristics and "terminal capacity" and "capacity" are defined herein as the measured electrical capacity between two terminals in an SCF circuit. The difference between these terms becomes significant in the following description and understanding of this invention which improves over the prior art switched capacitor circuits and provides for a much more efficient use of the stratum area than possible in the prior art circuits without sacrificing control of the filter characteristics.